What is the ruling regarding a mature person who has lost his mind? And will he be held accountable for the past, or will he not be held accountable after losing his mind?

An insane person is held accountable for their actions committed while sane. However, after losing their sanity, they are not held accountable for their actions, as they are no longer morally obligated. The Messenger of Allah, peace and blessings be upon him, said: "The pen has been lifted from three: from the insane person whose mind is overcome, from the sleeper until he awakens, and from the child until he reaches puberty." As for what an insane person damages of property, they are liable to compensate its owners, even if the sin is lifted from them.